The recent advent of the use of Automatic Dependent Surveillance—Broadcast (ADS-B), an advanced air ground traffic control system, has facilitated the integration of this system with the pre-existing Traffic Information System (TIS).
ADS-B is a technology which allows aircraft to broadcast information such as identification, position, altitude. This broadcast information may be directly received and processed by other aircraft or received and processed by ground systems for use in improved situational awareness, conflict avoidance and airspace management. ADS-B incorporates the use of Global Positioning System (GPS) or other similar navigation systems as a source of position data. By using GPS or the like, ADS-B has the capacity to greatly improve the efficiency and safety of the National Airspace System.
ADS-B provides for an automatic and periodic transmission of flight information from an in-flight aircraft to either other in-flight aircraft or ground systems. The ADS-B transmission will typically comprise information items such as altitude, flight ID, GPS (Global Positioning System) position, velocity, altitude rate, etc. The transmission medium for ADS-B can implement VHF, 1090 MHz (Mode S), UHF (UAT), or a combination of systems.
TIS is a technology in which air traffic control Secondary Surveillance Radar (SSR) on the ground transmits traffic information about nearby aircraft to any suitably equipped aircraft within the SSR range. The transmissions are addressed to a particular aircraft, and are sent together with altitude or identity interrogations. This lets an aircraft receive information about nearby aircraft, which do not have ADS-B capability, but are being interrogated by the SSR radar. The TIS information, like ADS-B information, is directed to a CDTI display for the benefit of the flight crew.
Traffic alert and Collision Avoidance Systems (TCAS) functionality can be improved with the GPS positioning capabilities of the ADS-B system. Such GPS position information will aid TCAS in determining more precise range and bearing at longer ranges. With greater precision, commercial aircraft can achieve higher safety levels and perform enhanced operational flying concepts such as in-trail climbs/descents, reduced vertical separation, and closely sequenced landings.
Additionally, ADS-B can also be used to extend traffic surveillance over greater distances. Previous technology limited surveillance ranges to a maximum of about 40 nautical miles (nm). ADS-B, since it does not require an active TCAS interrogation to determine range and bearing, will not be subject to a power limitation. As a result, in general, the ADS-B receiver capability determines surveillance range. For example, if the ADS-B receiver can process an ADS-B transmission out to 100 nm, then 100 nm is the effective range.
However, for ADS-B to be fully effective it must be implemented on both the aircraft transmitting and receiving ABS-B and all target aircraft within range. If one aircraft has ADS-B and the other does not, neither aircraft can achieve the full benefits of its use. Each aircraft remains “blind” to the other. For full implementation of ADS-B to occur all existing aircraft would require new technologies and equipment, including GPS sensors, some form of ADS-B transceiver, upgraded displays to present ADS-B target aircraft, and some form of data concentrator to collect and process all the appropriate ADS-B data. This would require most of the aircraft flying today to be extensively re-wired and re-equipped with new hardware.
As a result of the problems related to integrating ADS-B into the present fleet of aircraft, ADS-B equipped aircraft, as well as non-ADS-B equipped aircraft, must be capable of receiving positioning information from Traffic Information System (TIS) messages transmitted from ground stations. The ADS-B and TIS position information are processed in-flight, and the position of surrounding targets is displayed graphically on a cockpit display of traffic information (CDTI) unit located in each aircraft.
Because TIS information does not possess the same level of resolution quality as that of ADS-B and because of signal interference, it is possible that the traffic information for the same set of surrounding aircraft reported by TIS and ADS-B do not match. An on-board computer must correlate this conflicting traffic information and display one symbol (e.g., icon) on the CDTI for each actual aircraft. It is known that a suitable TIS/ADS-B correlation algorithm may be constructed based on the MIT Lincoln Lab's report 42PM-DataLink-0013 (hereafter referred to as the MIT Algorithm). The MIT Algorithm comprises essentially three steps:                1. Evaluate the similarity between every TIS target and every ADS-B target.        2. Store the evaluated similarities into a correlation array.        3. Correlate the TIS target with the ADS-B target that are similar and closest to each other.        
In step 1, the similarity between each TIS target and each ADS-B target is set as a binary logic function in which the bearing, range, relative altitude and track of each TIS and ADS-B target is compared to evaluate the similarity. Since binary logic rigidly produces the output of either yes (1) or no (0) to each comparison, it may fail to correlate two aircraft if only one single condition of the logic narrowly fails. For example, if one target makes a 45 degree turn according to ADS-B and a 47 degree turn according to TIS then the result is a no (0) in step 1 of the MIT algorithm and the targets are not correlated (i.e., two targets appear on the CDTI). This binary inflexibility significantly reduces the accuracy of the MIT algorithm, especially when targets are performing maneuvers. It is believed by those skilled in the art that the MIT algorithm may only produce a successful correlation rate of about 75 percent.
Therefore, an unresolved need exists for a more accurate and reliable method for correlating TIS and ADS-B target information.